Vibrational-amplitude variations

Vibration amplitude variations to be used during the search must be specified in a block starting with the

= VIB_DELTA

header flag, followed by a list of displacements for each of the atoms for which the vibrational amplitude should be varied during the search.

Example

= VIB_DELTA
O 1 = -0.05 0.05 0.02         ! Vibrational amplitude of oxygen atom 1 (and symmetry-equivalent atoms) will be varied over the range [-0.05, 0.05] with step 0.02
Ir_top = -0.05 0.05 0.01      ! Vibrational amplitude of Iridium atoms in Ir_top sites will be varied over the range [-0.05, 0.05] with step 0.01

How atoms are addressed on the left is described on the main DISPLACEMENTS page. The values on the right will be interpreted relative to the atom’s vibrational amplitude as defined in the VIBROCC file.

For some applications, it can be useful to apply a static displacement, without re-doing the reference calculation. For this purpose, the VIB_DELTA block also accepts single-value input on the right:

= VIB_DELTA
O 1 = 0.02         ! Vibrational amplitude of oxygen atom 1 (and symmetry-equivalent atoms) will be offset from the value in VIBROCC by 0.02

When multiple searches are executed consecutively or in a loop, the displacement ranges are per default centered around the optimized vibrational amplitude from previous searches per default. However, if you give a single-valued (static) displacement for an atom, the optimized vibrational amplitude from previous searches is discarded instead, and the static displacement is applied to its original position (to avoid “displacement creep” when the search is repeated). If you want to center the displacement range around the original vibrational amplitude of the atom, you can also clear the offset manually:

O 1 offset = 0         ! center range around original vibrational amplitude, instead of the optimized vibrational amplitude resulting from previous searches
O 1 offset = clear     ! equivalent
O 1 offset = original  ! equivalent

Note

  • The range of vibrational amplitudes defined with the command will be applied to all symmetry-equivalent atoms, unless you turn off symmetry constraints via SYMMETRY_FIX or The SYM_DELTA tag.

  • Vibrational amplitudes must be positive values, so the program will throw an error if the combination of the given range with the center point defined in the VIBROCC file results in negative values.

  • If your combination of start, end and step values does not yield an odd number of steps, the interval will be expanded symmetrically

around the midpoint.